OK. The test case does not check why this selector does not match, it only verifies that it doesn't match.
Ultimately, testing invalid scenarios e.g. bad grammar will require invalid CSS, which may also require invalid markup. I don't think that should be an issue since this is a test suite. Compiler and interpreter tests include a ton of files that do not compile. I would certainly expect a complete XML parser test suites to include invalid XML documents as well. We're not testing XHTML. We're testing CSS.
If lack of well-formedness impacts the build or test systems, we should talk about that.
And since invalid markup will prevent an XHTML-compliant browser to render the test, we should consider whether XHTML is the proper format for these test cases and/or the test suite.